In thermally stimulated continuous inkjet printing, see, for example, U.S. Pat. No. 6,588,888 B2, issued to Jeanmaire et al., on Jul. 8, 2003; and U.S. Pat. No. 6,079,821, issued to Chwalek et al., on Jun. 27, 2000, periodic heat pulses are applied to individual heaters embedded in a nozzle array. The periodic heat pulses drive capillary break-up of jets formed at each nozzle to produce an array of drops. The period of the pulse waveform determines the ultimate size of drop formed after jet break-up. Because the jet responds most sensitively to disturbances at a characteristic frequency fR known as the Rayleigh frequency, drops are most effectively produced at a fundamental size corresponding to a volume of fluid given by π2 U/fR, where r is the jet radius and U is the jet velocity.
U.S. Pat. No. 6,851,796 B2, issued to Jeanmaire et al., on Feb. 8, 2005, describes a printing system that relies on the ability to generate distinct sizes of drop—a “print drop” of a given size, and a “catch drop” of distinctly different size. Differential deflection of the drops of different sizes is employed to cause print drops to impinge on the substrate and the catch drops to be collected and recirculated through the ink delivery system. As described in U.S. Pat. No. 6,851,796 B2, an ink drop forming mechanism selectively creates a stream of ink drops having a plurality of different volumes traveling along a first path. A gas flow directed across the stream of ink drops interacts with the stream of ink drops. This interaction deflects smaller drops more than larger drops and thereby separates ink drops having one volume from ink drops having other volumes.
As the drop selection mechanism described above depends on drop size, it is necessary for large-volume drops to be fully formed before being exposed to the deflection air flow. Consider, for example, a case where the large-volume drop is to have a volume equal to four small-volume drops. It is often seen during drop formation that the portion of the ink stream that is to form the large-volume drop will separate from the main stream as desired, but will then break apart before coalescing to form the large-volume drop. It is necessary for this coalescence to be complete prior to passing through the drop deflecting air flow. Otherwise the separate fragments that are to form the large-volume drop will be deflected by an amount greater than that of a single large-volume drop. Similarly, the small-volume drops must not merge in air before having past the deflection air flow. If separate small-volume drops merge, they will be deflected less than desired.
The distance over which the large volume drop forms upon coalescence of its fragments is known as the drop formation length (DFL), denoted herein as LD. The details of the large drop waveform and the physical properties of the jet determine the size of LD. For the purposes of printing, smaller drop formation lengths are advantageous, as the drops are then available for size separation at distances closer to the nozzle plate, and the distance over which the drops must travel prior to separation is reduced. Thus a smaller drop formation length helps reduce the size of the print head and reduces the risk of incomplete large drop formation and reduces the risk of unintended merging of small drops.
It has been found that the small-volume drops between coalesced large-volume drops can be very unevenly spaced. In extreme circumstances, the large-volume drop often remains only partially formed until the large-volume drop is well beyond the deflection air flow. The partially formed large-volume drop and the small-volume drop immediately in front of it must merge to produce the completed large-volume drop. Occasionally, an undesirable merging of a small-volume drop and a large-volume drop will occur at some distance from the orifices. It is desirable to have the merging drops coalesce as quickly as possible after break off without additional merging of the small-volume drops with large-volume drops or with adjacent small-volume drops.
Continuous drop emission systems that utilize stimulation per jet apparatus are effective in providing control of the break-up parameters of an individual jet within a large array of jets. As described in U.S. Pat. No. 7,777,395 B2, issued to Xu et al., on Aug. 17, 2010, however, even when the stimulation is highly localized to each jet, for example, via resistive heating at the nozzle exit of each jet, some stimulation crosstalk still propagates as acoustic energy through the liquid via the common supply chambers. The added acoustic stimulation crosstalk from adjacent jets may adversely affect jet break up in terms of break-off timing or satellite drop formation. When operating in a printing mode of generating different predetermined drop volumes, according to the liquid pattern data, acoustic stimulation crosstalk may alter the jet break-up producing drops that are not the desired predetermined volume. Especially in the case of systems using multiple predetermined drop volumes, the effects of acoustic stimulation crosstalk are data-dependent, leading to complex interactions that are difficult to predict.
Stimulation crosstalk can manifest itself in a pattern along an entire nozzle array, suggestive of acoustic modes in portions of the printhead behind the nozzle array. In addition to the long-range effects including, for example, over hundreds to thousands of nozzles and macroscopic distances, there are short-range effects in which stimulation of a given jet affects neighboring jets. Of particular importance is the effect of producing a large drop in one jet while making small drops in neighboring jets. The disturbance resulting from the large drop waveform can impart differential velocity to small drops in a neighboring jet, thereby causing unintended merging of small drops. The degree of disturbance in neighboring jets caused by a large-drop waveform is sensitive to the details of the large-drop waveform. Large-drop waveforms wherein the heat pulses minimally disturb the neighboring jets concurrently operating during printing are advantageous, as high-quality prints are more readily achieved with simple and robust data processing algorithms requiring less compensation for particular patterns of drop formation in neighboring jets.
Thus, there is a need for waveforms for making large drops that provides a short drop formation length with reduced disturbance of neighboring jets.